Method for forming semiconductor structure

ABSTRACT

A method for forming a semiconductor structure includes forming a first FET device and a second FET device over a substrate. Forming a first gate trench in the first FET device and a second gate trench in the second FET device. Forming a first high-k gate dielectric layer in the first gate trench, and a second high-k gate dielectric layer in the second gate trench. Forming a first barrier layer over the first high-k gate dielectric layer, and a second barrier layer over the second high-k gate dielectric layer. Increasing N nitridations in the first and second barrier layers. Removing the second barrier layer to expose the second high-k gate dielectric layer. Forming a first work function metal layer over the first barrier layer and a second work function metal layer over the second high-k gate dielectric layer.

PRIORITY CLAIM AND CROSS-REFERENCE

This patent is a divisional application of U.S. Pat. Application Ser.No. 17/161,074, filed on Jan. 28, 2021, entitled of “SEMICONDUCTORSTRUCTURE AND METHOD FOR FORMING THE SAME”, which is incorporated byreference in its entirety.

BACKGROUND

The electronics industry has experienced an ever increasing demand forsmaller and faster electronic devices that are able to support greaternumbers of increasingly complex and sophisticated functions.Accordingly, there is a continuing trend in the semiconductor industryto manufacture low-cost, high-performance, low-power integrated circuits(ICs). Thus far these goals have been achieved in large part by scalingdown semiconductor IC dimensions (e.g., minimum feature size) andthereby improving production efficiency and reducing associated costs.However, such downscaling has also introduced increased complexity tothe semiconductor manufacturing process. Thus, the realization ofcontinued advances in semiconductor ICs and devices required similaradvances in semiconductor manufacturing processes and technology.

As technology nodes achieve progressively smaller scales, in some ICdesigns, researchers have hoped to replace a typical polysilicon gatewith a metal gate to improve device performance by decreasing featuresizes. One approach of forming the metal gate is called a “gate-last”approach, sometimes referred to as replacement polysilicon gate (RPG)approach. In the RPG approach, the metal gate is fabricated last, whichallows for a reduced number of subsequent operations.

Further, as the dimensions of a transistor decrease, the thickness ofthe gate dielectric layer may be reduced to maintain performance with adecreased gate length. In order to reduce gate leakage, a highdielectric constant (high-k or HK) gate dielectric layer is used toprovide a thickness as effective as that provided by a typical gateoxide used in larger technology nodes. A high-k metal gate (HKMG)approach including the metal gate electrode and the high-k gatedielectric layer is therefore recognized. However, the HKMG approach isa complicated approach, and many issues arise.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It shouldbe noted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a flowchart representing a method for forming a semiconductorstructure according to aspects of the present disclosure.

FIG. 2 shows perspective views illustrating portions of a semiconductorstructure at a fabrication stage according to aspects of the presentdisclosure in one or more embodiments.

FIG. 3 shows cross-sectional views taken along lines X1-X1′ and X2-X2′of FIG. 2 , respectively.

FIGS. 4 to 11 are schematic drawings illustrating the semiconductorstructure at various fabrication stages subsequent the stages shown into FIG. 3 according to aspects of the present disclosure in one or moreembodiments.

FIG. 12 shows perspective views illustrating portions of a semiconductorstructure at a fabrication stage according to aspects of the presentdisclosure in one or more embodiments.

FIG. 13 shows cross-sectional views taken along lines X1-X1′, X2-X2′ andX3-X3′ of FIG. 12 , respectively.

FIGS. 14 to 21 are schematic drawings illustrating the semiconductorstructure at various fabrication stages subsequent to the stage shown inFIG. 13 according to aspects of the present disclosure in one or moreembodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of elements and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper,” “on” and the like, may be used herein for ease ofdescription to describe one element or feature’s relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

As used herein, the terms such as “first,” “second” and “third” describevarious elements, components, regions, layers and/or sections, but theseelements, components, regions, layers and/or sections should not belimited by these terms. These terms may be only used to distinguish oneelement, component, region, layer or section from another. The termssuch as “first,” “second” and “third” when used herein do not imply asequence or order unless clearly indicated by the context.

Notwithstanding that the numerical ranges and parameters setting forththe broad scope of the disclosure are approximations, the numericalvalues set forth in the specific examples are reported as precisely aspossible. Any numerical value, however, inherently contains certainerrors necessarily resulting from the standard deviation found in therespective testing measurements. Also, as used herein, the terms“substantially,” “approximately” or “about” generally mean within avalue or range that can be contemplated by people having ordinary skillin the art. Alternatively, the terms “substantially,” “approximately” or“about” mean within an acceptable standard error of the mean whenconsidered by one of ordinary skill in the art. People having ordinaryskill in the art can understand that the acceptable standard error mayvary according to different technologies. Other than in theoperating/working examples, or unless otherwise expressly specified, allof the numerical ranges, amounts, values and percentages such as thosefor quantities of materials, durations of times, temperatures, operatingconditions, ratios of amounts, and the likes thereof disclosed hereinshould be understood as modified in all instances by the terms“substantially,” “approximately” or “about.” Accordingly, unlessindicated to the contrary, the numerical parameters set forth in thepresent disclosure and attached claims are approximations that can varyas desired. At the very least, each numerical parameter should at leastbe construed in light of the number of reported significant digits andby applying ordinary rounding techniques. Ranges can be expressed hereinas from one endpoint to another endpoint or between two endpoints. Allranges disclosed herein are inclusive of the endpoints, unless specifiedotherwise.

With the ongoing down-scaling of integrated circuits, power supplyvoltages of the circuits may be reduced. However, the voltage reductionmay be different in different circuits or regions. For example,threshold voltage (Vt) requirements may be different between the memorycircuits and the core circuits. A multiple-Vt capability is thereforerequired for device design.

Further, as the gate length (Lg) scale is reduced in advanced nodes, torealize the multiple-Vt design using different gate metal materialsbecomes challenging due to the limited Lg and the gap-filling abilityrequirements.

Embodiments of a method for forming a semiconductor structure aretherefore provided. The semiconductor structure is formed in an HKMGprocess in accordance with the embodiments. The semiconductor structurecan be formed in a planar device process according to some embodiments.The semiconductor structure can be formed in a non-planar device inalternative embodiments. In some embodiments, the method for forming thesemiconductor structure includes in-situ and/or ex-situ nitridation onbarrier layers. The barrier layers with different nitrogen (N)concentrations may have different metal barrier abilities. In someembodiments, when the metal includes aluminum (Al), the barrierincluding greater N concentration provides greater Al-diffusion barrierability. Further, the greater Al-diffusion barrier ability is requiredby higher-voltage devices. Accordingly, the method provides barrierswith different N concentrations to meet multiple-Vt structurerequirements.

FIG. 1 is a flowchart representing a method for forming a semiconductorstructure 10 according to aspects of the present disclosure. The method10 includes a number of operations (101, 102, 103, 104, 105, 106, and107). The method 10 will be further described according to one or moreembodiments. It should be noted that the operations of the method 10 maybe rearranged or otherwise modified within the scope of the variousaspects. It should be further noted that additional processes may beprovided before, during, and after the method 10, and that some otherprocesses may just be briefly described herein. Thus, otherimplementations are possible within the scope of the various aspectsdescribed herein.

FIG. 2 shows perspective views illustrating portions of a semiconductorstructure according to aspects of the present disclosure. In someembodiments, in operation 101, the method 10 include forming a first FETdevice 210 a and a second FET device 210 b over a substrate 200. In someembodiments, the substrate 200 may be a semiconductor substrate such asa silicon substrate. The substrate 200 may also include othersemiconductors such as germanium (Ge), silicon carbide (SiC), silicongermanium (SiGe), or diamond. Alternatively, the substrate 200 mayinclude a compound semiconductor and/or an alloy semiconductor. Thesubstrate 200 may include various layers, including conductive orinsulating layers formed on a semiconductor substrate. The substrate 200may include various doping configurations depending on designrequirements, as is known in the art. For example, different dopingprofiles (e.g., n wells or p wells) may be formed on the substrate 200in regions designed for different device types (e.g., n-typefield-effect transistors (NFET), or p-type field-effect transistors(PFET)). The suitable doping may include ion implantation of dopantsand/or diffusion processes.

In some embodiments, the substrate 200 may include a first region 202 aand a second region 202 b. Further, the substrate 200 may includeisolation structures, e.g., shallow trench isolation (STI) structures204 interposing the first and second regions 202 a and 202 b. The firstand second regions 202 a and 202 b are defined for accommodatingdifferent devices. For example, the first region 202 a may accommodate ahigh voltage (HV) device while the second region 202 b may accommodate alow voltage (LV) device. In some embodiments, the HV device used hereinis a device having an operating voltage greater than that of the LVdevice. However, operating voltages can vary for different applications,thus they are not limited herein.

In some embodiments, the devices 210 a and 210 b may be planartransistors or multi-gate transistors, such as fin-like FETs (FinFETs).

In some embodiments, in operation 101, the first FET device 210 a isformed in the first region 202 a. In some embodiments, the first FETdevice 210 a may be an HV device. In some embodiments, the first FETdevice 210 a may an n-type HV device, but the disclosure is not limitedthereto. The first FET device 210 a may include a first gate structure212 a and a first source/drain 214 a. In some embodiments, the first FETdevice 210 a may be a first FinFET device, and a first fin structure 206a is disposed over the substrate 200, as shown in FIG. 2 . A portion ofthe first fin structure 206 a covered by the first gate structure 212 aserves as a channel region, and portions of the first fin structure 206a exposed through the first gate structure 212 a serve as the firstsource/drain 214 a.

In operation 101, the second FET device 210 b is formed in the secondregion 202 b. In some embodiments, the second FET device 210 b may be anLV device. In some embodiments, the second FET device 210 a may be ann-type LV device, but the disclosure is not limited thereto. The secondFET device 210 b may include a second gate structure 212 b and a secondsource/drain 214 b. In some embodiments, the second FET device 210 b isa second FinFET device, and a second fin structure 206 b is disposedover the substrate 200, as shown in FIG. 2 . Similar to the first FETdevice 210 a described above, in the second FET device 210 b, a portionof the second fin structure 206 b covered by the second gate structure212 b serves as a channel region, and portions of the second finstructure 206 b exposed through the second gate structure 212 b serve asthe second source/drain 214 b.

In some embodiments, the first gate structure 212 a and the second gatestructure 212 b are sacrificial gate structures. The first and secondsacrificial gate structures may respectively include a dielectric layerand a sacrificial semiconductor layer. In some embodiments, thesemiconductor layers are made of polysilicon, but the disclosure is notlimited thereto. In some embodiments, spacers 216 (shown in FIG. 3 ) canbe formed over sidewalls of the sacrificial gate structures. In someembodiments, the spacers 216 are made of silicon nitride (SiN), siliconcarbide (SiC), silicon oxide (SiO), silicon oxynitride (SiON), siliconcarbide or any other suitable material, but the disclosure is notlimited thereto. In some embodiments, the spacers 216 are formed bydeposition and etch back operations.

As shown in FIG. 2 , in some embodiments, the first source/drain 214 ais formed over the first fin structure 206 a at two opposite sides ofthe first gate structure 212 a. Similarly, the second source/drain 214 bis formed over the second fin structure 206 b at two opposite sides ofthe second gate structure 212 b. In some embodiments, heights of thefirst source/drain 214 a and the second source/drain 214 b may begreater than heights of the first and second fin structures 206 a and206 b. In some embodiments, the first and second source/drain 214 a and214 b may be formed by forming recesses in the fin structures 206 a and206 b and growing a strained material in the recesses by an epitaxial(epi) process. In addition, the lattice constant of the strainedmaterial may be different from the lattice constant of the finstructures 206 a and 206 b. Accordingly, the first and secondsource/drain 214 a and 214 b may serve as stressors that improve carriermobility. In some embodiments, the first source/drain 214 a and thesecond source/drain 214 b may both include n-type dopants. However, adopant concentration of the first source/drain 214 a may be differentfrom that of the second source/drain 214 b.

In some embodiments, after the forming of the source/drain structures, acontact etch stop layer (CESL) 218 may be formed to cover the first andsecond gate structures 212 a and 212 b over the substrate 200. In someembodiments, the CESL 218 can include silicon nitride, siliconoxynitride, and/or other applicable materials. Subsequently, aninter-layer dielectric (ILD) structure 220 may be formed on the CESL 218in accordance with some embodiments. The ILD structure 220 may includemultilayers made of multiple dielectric materials, such as siliconoxide, silicon nitride, silicon oxynitride, tetraethoxysilane (TEOS),phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), low-kdielectric material, and/or other applicable dielectric materials.Examples of low-k dielectric materials include, but are not limited to,fluorinated silica glass (FSG), carbon-doped silicon oxide, amorphousfluorinated carbon, parylene, bis-benzocyclobutenes (BCB), or polyimide.Next, a polishing process is performed on the ILD structure 220 and theCESL 218 to expose top surfaces of the first and second gate structures212 a and 212 b, as shown in FIG. 3 . In some embodiments, the ILDstructure 220 and the CESL 218 are planarized by a chemical mechanicalpolishing (CMP) process until the top surfaces of the first and secondgate structures 212 a and 212 b are exposed. Consequently, the ILDstructure 220 surrounds the first and second gate structures 212 a, 212b and the first and second fin structures 206 a, 206 b. In other words,the fin structures 206 a, 206 b and the sacrificial gate structures 212a, 212 b are embedded in the ILD structure 220, while a top surface ofthe sacrificial gate structures 212 a, 212 b remains exposed, as shownin FIG. 3 .

Referring to FIG. 4 , in some embodiments, in operation 102, the method10 includes forming a first gate trench 221 a in the first FET device210 a and a second gate trench 221 b in the second FET device 210 b. Insome embodiments, the sacrificial semiconductor layer is removed. Insome embodiments, the dielectric layer may be removed for forming aninterfacial layer (IL). In some embodiments, the dielectric layer may beleft in the gate trench, though not shown. It should be noted that theremoval of the dielectric layer may be performed depending on differentprocess or product requirements. Accordingly, the first fin structure206 a is exposed through the first gate trench 221 a, and the second finstructure 206 b is exposed through the second gate trench 221 b, asshown in FIG. 4 . Additionally, in some embodiments, a protecting capmay be formed over the ILD structure 220. The protecting cap may includea material different from that of the ILD structure 220. The protectingcap protects the ILD structure 220 during the removing of thesacrificial semiconductor layer and other subsequent operations.

Referring to FIG. 5 , in some embodiments, in operation 103, the method10 includes forming a first high-k gate dielectric layer 222 a in thefirst gate trench 221 a and a second high-k gate dielectric layer 222 bin the second gate trench 221 b. A thickness of the first high-k gatedielectric layer 222 a and a thickness of the second high-k gatedielectric layer 222 b are similar. In some embodiments, the thicknessesof the first and second high-k gate dielectric layers 222 a and 222 bmay be between approximately 1 nanometer and approximate 3 nanometers,but the disclosure is not limited thereto. In some embodiments, an ILlayer may be formed prior to the forming of the first and second high-kgate dielectric layers 222 a and 222 b, though not shown. The IL layermay include an oxide-containing material such as SiO or SiON. In someembodiments, the IL layer covers portions of the fin structures 206 a,206 b exposed in the gate trenches 221 a, 221 b. The first and secondhigh-k gate dielectric layers 222 a and 222 b may be simultaneouslyformed on the IL layer. In some embodiments, the first and second high-kgate dielectric layers 222 a and 222 b may be conformally formed in thegate trenches 221 a and 221 b. Accordingly, the first high-k gatedielectric layer 222 a covers at least sidewalls of the first gatetrench 221 a, and the second high-k gate dielectric layer 222 b coversat least sidewalls of the second gate trench 221 b. In some embodiments,the first and second high-k gate dielectric layers 222 a and 222 binclude a high-k dielectric material having a high dielectric constant,for example, greater than that of thermal silicon oxide (^(~)3.9). Thehigh-k dielectric material may include hafnium oxide (HfO₂), zirconiumoxide (ZrO₂), lanthanum oxide (La₂O₃), aluminum oxide (Al₂O₃), titaniumoxide (TiO₂), yttrium oxide (Y₂O₃), strontium titanate (SrTiO₃), hafniumoxynitride (HfOxNy), other suitable metal-oxides, or combinationsthereof.

Referring to FIG. 6 , in some embodiments, in operation 104, the method10 includes forming a first barrier layer 224 a over the first high-kgate dielectric layer 222 a and a second barrier layer 224 b over thesecond high-k gate dielectric layer 222 b. In some embodiments, thefirst and second barrier layers 224 a and 224 b are formedsimultaneously. In some embodiments, the first and second barrier layers224 a and 224 b may be conformally formed in the gate trenches 221 a and221 b. A thickness of the first barrier layer 224 a and a thickness ofthe second barrier layer 224 b are similar. In some embodiments, thethicknesses of the first and second barrier layers 224 a and 224 b maybe between approximately 0.1 nanometer and approximately 10 nanometers,but the disclosure is not limited thereto. For example, the thicknessesof the first and second barrier layers 224 a and 224 b may be less thanapproximately 1.5 nanometer. Further, the first and second barrierlayers 224 a and 224 b include a same material. In some embodiments, thefirst and second barrier layers 224 a and 224 b both include tungsten(W). For example, the first and second barrier layers 224 a and 224 bmay include W-based metal, such as WNx, WCx, WCxNy, W-based metal withoxygen, W-based metal without oxygen, or combinations thereof. Further,a W concentration in each of the first and second barrier layers 224 aand 224 b is between approximately 10% and approximately 70%, but thedisclosure is not limited thereto.

In some embodiments, in operation 105, the method 10 includes increasinga nitrogen (N) concentration in the first barrier layer 224 a and an Nconcentration in the second barrier layer 224 b. Further, the increasingof the N concentrations includes an in-situ treatment and/or an ex-situtreatment. In some embodiments, the N concentration in the first andsecond barrier layers 224 a and 224 b may be increased by the in-situtreatment. The in-situ treatment includes a prolonged ammonia (NH₃)pulse during the forming of the first and second barrier layers 224 aand 224 b and/or a post-ammonia soak. In some embodiments, a depositionmethod may be used to form the first and second barrier layers 224 a and224 b, and a variable supply of nitrogen-excited species through aremote plasma generator may be involved. By controlling the pulseduration, pulse condition, flow rate, and therefore chemicalavailability, the N concentration of the first and second barrier layers224 a and 224 b can be adjusted. In some embodiments, a post-NH₃ soakmay be used in a thermal environment or a plasma environment. Bycontrolling soak duration, soak condition, and therefore chemicalavailability, the N concentration of the first and second barrier layers224 a and 224 b can be adjusted. In some embodiments, the W-N bondingcan be enhanced in the NH₃ environment or by plasma. In some comparativeapproaches, when the W-containing barrier layers 224 a and 224 b areexposed to air, tungsten oxide tends to be formed. However, the in-situtreatment helps to form more robust W-N bonding for better oxidationresistance, thus mitigating the oxidation issue. In other words,oxidation immunity of the barrier layers 224 a and 224 b may be improvedby the in-situ treatment.

Referring to FIG. 7 , in some embodiments, the N concentration in thefirst and second barrier layers 224 a and 224 b may be increased by theex-situ treatment. In some embodiments, the ex-situ treatment may beperformed after the in-situ treatment. In other embodiments, the ex-situtreatment may be performed directly after the forming of the first andsecond barrier layers 224 a and 224 b. In still other embodiments, theex-situ treatment may be omitted. The ex-situ treatment may include anitridation 223 using NH₃, nitrogen (N₂), triatomic hydrogen (H₃), inertgas, or combinations thereof. In some embodiments, the nitridation 223may be performed at a temperature between approximately 200° C. andapproximately 600° C., in a pressure between approximately 1mTorr andapproximately 100 mTorr, and in a flow rate between approximately 1standard cubic centimeters per minute (sccm) and approximately 100standard liters per minute (slm or slpm). As mentioned above, in somecomparative approaches when the W-containing barrier layers 224 a and224 b are exposed to air, tungsten oxide tends to be formed. However,the ex-situ treatment helps to reduce surface oxidation and re-form W-Nbonding, and thus further mitigates the oxidation issue and improvesoxidation immunity.

Accordingly, the first and second barrier layers 224 a and 224 b may bereferred to as N-containing barrier layers after the increasing of the Nconcentration. Further, the N concentration in each of the first andsecond N-containing barrier layers 224 a and 224 b may be increased tobetween approximately 10% and approximately 70%, but the disclosure isnot limited thereto.

Referring to FIG. 8 , in some embodiments, in operation 106, the method10 includes removing the second barrier layer 224 b to expose the secondhigh-k gate dielectric layer 222 b in the second gate trench 221 b. Insome embodiments, a protecting layer or a masking layer may be formed inthe first region 202 b, and a suitable etching operation may beperformed to remove the second barrier layer 224 b. Thus, the secondbarrier layer 224 b is removed from the second region 202 b. Theprotecting layer or the masking layer is removed after the removing ofthe second barrier layer 224 b.

Referring to FIG. 9 , in some embodiments, in operation 107, the method10 includes forming a first work function metal layer 226 a over thefirst barrier layer 224 a and a second work function metal layer 226 bover the second high-k gate dielectric layer 222 b. A thickness of thefirst work function metal layer 226 a and a thickness of the second workfunction metal layer 226 b may be similar. In some embodiments, thethicknesses of the first and second work function metal layers 226 a and226 b may be between approximately 0.5 nanometer and approximately 5nanometers, but the disclosure is not limited thereto. In someembodiments, the first work function metal layer 226 a may be in directcontact with the first barrier layer 224 a, while the second workfunction metal layer 226 b may be in direct contact with the secondhigh-k gate dielectric layer 222 b. A thickness of the first workfunction metal layer 226 a and a thickness of the second work functionmetal layer 226 b may be similar. The first work function metal layer226 a and the second work function metal layer 226 b may both be n-typework function metal layers. Further, the first and second work functionmetal layers 226 a and 226 b may include same n-type metal materials. Insome embodiments, the first work function metal layer 226 a and thesecond work function metal layer 226 b may both be n-type work functionmetal layers including aluminum (Al). In some embodiments, the first andsecond work function metal layers 226 a and 226 b may be single-layeredstructures or multilayers of two or more materials, but the disclosureis not limited thereto. In some embodiments, an Al-containing n-typemetal layer may be the layer closest to the first barrier layer 224 aand the second high-k gate dielectric layer 222 b.

Referring to FIG. 10 , in some embodiments, a gap-filling metal layer228 is formed to fill the first gate trench 221 a and the second gatetrench 221 b. In some embodiments, the gap-filling metal layer 228 caninclude conductive material such as Al, Cu, AlCu, or W, but is notlimited to the above-mentioned materials.

Referring to FIG. 11 , in some embodiments, a planarization operationsuch as a CMP may be performed to remove superfluous layers.Accordingly, portions of the first and second high-k gate dielectriclayers 222 a and 222 b, portions of the first barrier layer 224 a,portions of the first and second work function metal layers 226 a and226 b, and portions of the gap-filling layer 228 are removed. Thus, afirst metal gate structure 230 a is formed in the first FET device 210 ain the first region 202 a, and a second metal gate structure 230 b isformed in the second FET device 210 b in the second region 202 b. Insome embodiments, a top surface of the first metal gate structure 230 a,a top surface of the second metal gate structure 230 b and top surfacesof the ILD structure 220 may be aligned with each other (i.e., the topsurfaces may be co-planar).

Accordingly, a semiconductor structure 20 is obtained as shown in inFIG. 11 . The semiconductor structure 20 includes the first FET device210 a and the second FET device 210 b. As mentioned above, the first FETdevice 210 a and the second FET device 210 b may both be FinFET devices.Therefore, the first FET device 210 a includes the first metal gatestructure 230 a over the first fin structure 206 a, and the second FETdevice 210 b includes the second metal gate structure 230 b over thesecond fin structure 206 b. The first metal gate structure 230 aincludes the first high-k gate dielectric layer 222 a, the first workfunction metal layer 226 a over the first high-k gate dielectric layer222 a, and the first barrier layer 224 a between the first work functionmetal layer 226 a and the first high-k gate dielectric layer 222 a. Insome embodiments, the N concentration in the first barrier layer 224 ais between approximately 10% and approximately 70%. Therefore, the firstbarrier layer 224 a is referred to as an N-containing barrier layer. Thesecond metal gate structure 230 b includes the second high-k gatedielectric layer 222 b and the second work function metal layer 226 bover the second high-k gate dielectric layer 222 b. As mentioned above,the first and second work function metal layers 226 a and 226 b may ben-type work function metal layers. Further, the first and second workfunction metal layers 226 a and 226 b may be Al-containing n-type workfunction metal layers.

In some embodiments, it is found that Al may diffuse from the workfunction metal layers 226 a and 226 b. As shown in FIG. 11 , because thesecond work function metal layer 226 b is in contact with the secondhigh-k gate dielectric layer 222 b, Al may diffuse into the secondhigh-k gate dielectric layer 222 b. Consequently, the second high-k gatedielectric layer 222 b may include metal material, such as Al.Similarly, Al may diffuse from the first work function metal layer 226a. However, because the first barrier layer 224 a is disposed betweenthe first work function metal layer 226 a and the first high-k gatedielectric layer 222 a, the first barrier layer 224 a may mitigate theAl diffusion. It should be noted that in some comparative approaches, abarrier layer with a thickness the same as that of the first barrierlayer 224 a is not sufficient to mitigate the Al diffusion. In contrastto the comparative approaches, the method 10 includes increasing thenitrogen concentration in the first barrier layer 224 a such that, asmentioned above, the first barrier layer 224 a is referred to as anN-containing barrier layer. Nitrogen in the N-containing barrier layer224 a helps to obstruct Al diffusion even in such a thin layer (i.e., alayer having a thickness less than 1.5 nanometer). Therefore, Aldiffusion into the first high-k gate dielectric layer 222 a is less thanthat into the second high-k gate dielectric layer 222 b. In someembodiments, the first high-k gate dielectric layer 222 a may stillinclude the metal material such as Al, but a metal concentration (i.e.,the Al concentration) of the first high-k gate dielectric layer 222 a isless than a metal concentration (i.e., the Al concentration) of thesecond high-k gate dielectric layer 222 b.

It should be noted that the second high-k gate dielectric layer 222 bwith greater Al concentration is suitable for an LV device, and thefirst high-k gate dielectric layer 222 b with a lower Al concentrationis suitable for an HV device. Accordingly, the semiconductor structure20 is a multiple-Vt structures suitable for device design.

Accordingly, the method 10 includes forming the N-containing barrierlayer 224 a by in-situ and/or ex-situ treatment for the HV device. TheN-containing barrier layer 224 a is able to mitigate the Al diffusionwith a relatively thinner profile. Further, such thin barrier layer 224a renders less impact on gap filling. In short, the method 10 providesthe N-containing barrier in order to meet multiple-Vt structurerequirements with competitive gap-filling ability.

FIG. 12 shows perspective views illustrating portions of a semiconductorstructure according to aspects of the present disclosure, FIG. 13 showscross-sectional views taken along lines X1-X1′, X2-X2′ and X3-X3′ ofFIG. 12 , respectively, and FIGS. 14 to 21 are schematic drawingsillustrating the semiconductor structure at various fabrication stagessubsequent to the stage shown in FIG. 13 according to aspects of thepresent disclosure in one or more embodiments. It should be noted thatsame elements in FIGS. 2 to 11 and FIGS. 12 to 21 are indicated by samenumerals, and can include a same material. Thus, repeated descriptionsof details are omitted for brevity.

In some embodiments, in operation 101, the method 10 includes forming afirst FET device 210 a, a second FET device 210 b and a third FET device210 c over a substrate 200. As mentioned above, the substrate 200 mayinclude regions designed for different device types. For example, thesubstrate 200 may include a first region 202 a, a second region 202 band a third region 202 c. Further, the substrate 200 may includeisolation structures, e.g., STI structures 204, interposing the regions202 a, 202 b and 202 c. The regions 202 a, 202 b and 202 c are definedfor accommodating different devices. For example, the first region 202 amay accommodate an HV device, the second region 202 b may accommodate anLV device, and the third region 202 c may accommodate a middle voltage(MV) device. In some embodiments, the HV device used herein is a devicehave an operating voltage greater than that of the LV device, and the MVdevice is a device have an operating voltage between those of the HVdevice and the LV device. As mentioned above, operating voltages canvary for different applications, thus they are not limited herein.Additionally, the region arrangement is not limited by FIGS. 12 to 21 .

As mentioned above, the devices 210 a to 210 c may be planar transistorsor multi-gate transistors, such as FinFETs. The first and second FETdevices 210 a and 210 b are similar to those described above; therefore,repeated descriptions of details are omitted for brevity.

In some embodiments, in operation 101, the third FET device 210 c isformed in the third region 202 c. In some embodiments, the third FETdevice 210 c may be an MV device. In some embodiments, the third FETdevice 210 c may an n-type MV device, but the disclosure is not limitedthereto. The third FET device 210 c may include a third gate structure212 c and a third source/drain 214 c. In some embodiments, the third FETdevice 210 c may be a third FinFET device, and a third fin structure 206c is disposed over the substrate 200, as shown in FIG. 12 . The thirdgate structure 212 c is a sacrificial gate structure. As mentionedabove, the sacrificial gate structure may include a dielectric layer anda sacrificial semiconductor layer. As also mentioned above, spacers 216(shown in FIG. 13 ) can be formed over sidewalls of the sacrificial gatestructure 212 c.

As shown in FIG. 12 , the third source/drain 214 c is formed over thethird fin structure 206 c at two opposite sides of the third gatestructure 212 c in accordance with some embodiments. In someembodiments, a height of the third source/drain 214 c may be greaterthan a height of the fin structure 206 c. In some embodiments, the thirdsource/drain 214 c may include strained material serving as stressorsthat improve carrier mobility. In some embodiments, both of the thirdsource/drain 214 c may include n-type dopants, and a dopantconcentration of the third source/drain 214 c may be different fromthose of the first source/drain 214 a and the second source/drain 214 b.

As mentioned above, a CESL 218 may be formed to cover the sacrificialgate structures 212 a, 212 b and 212 c over the substrate 200.Subsequently, an ILD structure 220 may be formed on the CESL 218. Apolishing process may be performed on the ILD structure 220 and the CESL218 to expose top surfaces of the sacrificial gate structures 212 a, 212b and 212 c, as shown in FIG. 13 .

Referring to FIG. 14 , in some embodiments, in operation 102, the method10 includes forming a first gate trench 221 a in the first FET device210 a, a second gate trench 221 b in the second FET device 210 b, and athird gate trench 221 c in the third FET device 210 c. As mentionedabove, a removal of the dielectric layer may be performed depending ondifferent process or product requirements. In some embodiments, thefirst fin structure 206 a is exposed through the first gate trench 221a, the second fin structure 206 b is exposed through the second gatetrench 221 b, and the third fin structure 206 c is exposed through thethird gate trench 221 c, as shown in FIG. 14 .

Referring to FIG. 15 , in some embodiments, in operation 103, the method10 includes forming a first high-k gate dielectric layer 222 a in thefirst gate trench 221 a, a second high-k gate dielectric layer 222 b inthe second gate trench 221 b, and a third high-k gate dielectric layer222 c in the third gate trench 221 c. In some embodiments, an IL layermay be formed prior to the forming of the high-k gate dielectric layers222 a, 222 b and 222 c, though not shown. In some embodiments, thehigh-k gate dielectric layers 222 a, 222 b and 222 c may besimultaneously and conformally formed in the gate trenches 221 a, 221 band 221 c, respectively.

Referring to FIG. 16 , in some embodiments, in operation 104, the method10 includes forming a first barrier layer 224 a on the first high-k gatedielectric layer 222 a, a second barrier layer 224 b on the secondhigh-k gate dielectric layer 222 b, and a third barrier layer 224 c onthe third high-k gate dielectric layer 222 c. In some embodiments, thebarrier layers 224 a, 224 b and 224 c are formed simultaneously. In someembodiments, the barrier layers 224 a, 224 b and 224 c may beconformally formed in the gate trenches 221 a, 221 b and 221 c,respectively. A thickness of the first barrier layer 224 a, a thicknessof the second barrier layer 224 b and a thickness of the third barrierlayer 224 c are similar. In some embodiments, the thicknesses of thebarrier layers 224 a, 224 b and 224 c may be between approximately 0.1nanometer and approximately 10 nanometers, but the disclosure is notlimited thereto. For example, the thicknesses of the barrier layers 224a, 224 b and 224 c may be less than approximately 1.5 nanometer.Further, the barrier layers 224 a, 224 b and 224 c include a samematerial. In some embodiments, the barrier layers 224 a, 224 b and 224 cinclude tungsten. For example, the barrier layers 224 a, 224 b and 224 cmay include W-based metal, such as WNx, WCx, WCxNy, W-based metal withoxygen, W-based metal without oxygen, or combinations thereof. Further,a W concentration in each of the barrier layers 224 a, 224 b and 224 cis between approximately 10% and approximately 70%, but the disclosureis not limited thereto.

In some embodiments, in operation 105, the method 10 includes increasinga nitrogen concentration in each of the barrier layers 224 a, 224 b and224 c. As mentioned above, the increasing of the N concentrationsincludes an in-situ treatment and/or an ex-situ treatment. In someembodiments, the N concentration in each of the barrier layers 224 a,224 b and 224 c may be increased by the in-situ treatment. The in-situtreatment may be similar to those described above; therefore, repeateddescription of the in-situ treatment is omitted for brevity.

Referring to FIG. 17 , in some embodiments, the N concentration in eachof the barrier layers 224 a, 224 b and 224 c may be increased by theex-situ treatment. In some embodiments, the ex-situ treatment may beperformed after the in-situ treatment. In other embodiments, the ex-situtreatment may be performed directly after the forming of the barrierlayers 224 a, 224 b and 224 c. The ex-situ treatment may include a firstnitridation 223-1 using NH₃, N₂, H₃, inert gas, or combinations thereof.Parameters of the first nitridation 223-1 may be similar to thosedescribed above; therefore, repeated description of the ex-situtreatment is omitted for brevity.

Referring to FIG. 18 , in some embodiments, in operation 105, the method10 may include performing a second nitridation 223-2 to further increasethe N concentration. In some embodiments, a masking layer or protectinglayer 225 may be formed to cover the second region 202 b and the thirdregion 202 c. The second nitridation 223-2 may use NH₃, N₂, H₃, inertgas, or combinations thereof. Parameters of the second nitridation 223-2may be similar to those described above; therefore, repeated descriptionof the ex-situ treatment is omitted for brevity. In some embodiments,the N concentration of the first barrier layer 224 a may be furtherincreased, and thus the N concentration of the first barrier layer 224 ais greater than the N concentrations of the second barrier layer 224 band the third barrier layer 224 c. Additionally, the masking layer orthe protecting layer 225 is removed after the second nitridation.

Accordingly, the first, second and third barrier layers 224 a, 224 b and224 c may be referred to as N-containing barrier layers after theincreasing of the N concentration. The N concentration in each of theN-containing barrier layers 224 a, 224 b and 224 c may be increased tobetween approximately 10% and approximately 70%, but the disclosure isnot limited thereto. Further, the N concentration of the first barrierlayer 224 a is greater than the N concentrations of the second barrierlayer 224 b and the third barrier layer 224 c due to the two ex-situtreatments.

Referring to FIG. 19 , in some embodiments, in operation 106, the method10 includes removing the second barrier layer 224 b to expose the secondhigh-k gate dielectric layer 222 b in the second gate trench 221 b. Insome embodiments, a protecting layer or a masking layer (not shown) maybe formed in the first region 202 a and the third region 202 c, and asuitable etching operation may be performed to remove the second barrierlayer 224 b. Thus, the second barrier layer 224 b is removed from thesecond region 202 b. The protecting layer or the masking layer isremoved after the removing of the second barrier layer 224 b.

Referring to FIG. 20 , in some embodiments, in operation 107, the method10 includes forming a first work function metal layer 226 a over thefirst barrier layer 224 a, a second work function metal layer 226 b overthe second high-k gate dielectric layer 222 b, and a third work functionmetal layer 226 c over the third barrier layer 224 c. In someembodiments, the first work function metal layer 226 a may be in directcontact with the first barrier layer 224 a, and the third work functionmetal layer 226 c may be in direct contact with the third barrier layer224 c. In contrast with the first and third work function metal layers226 a and 226 c, the second work function metal layer 226 b may be indirect contact with the second high-k gate dielectric layer 222 b. Athickness of the first work function metal layer 226 a, a thickness ofthe second work function metal layer 226 b and a thickness of the thirdwork function metal layer 226 c may be similar. The work function metallayers 226 a, 226 b and 226 c may all be n-type work function metallayers. Further, the work function metal layers 226 a, 226 b and 226 cmay include same n-type metal materials. In some embodiments, the workfunction metal layers 226 a, 226 b and 226 c may all be n-type workfunction metal layers that includes aluminum.

Referring to FIG. 21 , in some embodiments, a gap-filling metal layer228 is formed to fill the first gate trench 221 a, the second gatetrench 221 b and the third gate trench 221 c.

Referring to FIG. 21 , in some embodiments, a planarization operationsuch as a CMP may be performed to remove superfluous layers.Accordingly, a first metal gate structure 230 a is formed in the firstFET device 210 a in the first region 202 a, a second metal gatestructure 230 b is formed in the second FET device 210 b in the secondregion 202 b, and a third metal gate structure 230 c is formed in thethird FET device 210 c in the third region 202 c. In some embodiments, atop surface of the first metal gate structure 230 a, a top surface ofthe second metal gate structure 230 b, a top surface of the third metalgate structure 230 c, and top surfaces of the ILD structure 220 may bealigned with each other (i.e., the top surfaces may be co-planar).

Accordingly, a semiconductor structure 20′ is obtained as shown in FIG.21 . The semiconductor structure 20′ includes the first FET device 210a, the second FET device 210 b and the third FET device 210 c. Asmentioned above, the FET devices 210 a, 210 b and 210 c may all beFinFET devices. Therefore, the first FET device 210 a includes the firstmetal gate structure 230 a over the first fin structure 206 a, thesecond FET device 210 b includes the second metal gate structure 230 bover the second fin structure 206 b, and the third FET device 210 cincludes the third metal gate structure 230 c over the third finstructure 206 c. The first metal gate structure 230 a includes the firsthigh-k gate dielectric layer 222 a, the first work function metal layer226 a, and the first barrier layer 224 a between the first work functionmetal layer 226 a and the first high-k gate dielectric layer 222 a. Insome embodiments, the N concentration in the first barrier layer 224 ais between approximately 10% and approximately 70%. Therefore, the firstbarrier layer 224 a is referred to as an N-containing barrier layer. Thethird metal gate structure 230 c includes the third high-k gatedielectric layer 222 c, the third work function metal layer 226 c, andthe third barrier layer 224 c between the third work function metallayer 226 c and the third high-k gate dielectric layer 222 c. In someembodiments, the N concentration in the third barrier layer 224 c isbetween approximately 10% and approximately 70%. Therefore, the thirdbarrier layer 224 c is also referred to as an N-containing barrierlayer. However, the N concentration of the first barrier layer 224 a isgreater than the N concentration of the third barrier layer 224 c. Thesecond metal gate structure 230 b includes the second high-k gatedielectric layer 222 b and the second work function metal layer 226 b.As mentioned above, the work function metal layers 226 a, 226 b and 226c may be n-type work function metal layers. Further, the work functionmetal layers 226 a, 226 b and 226 c may be Al-containing n-type workfunction metal layers.

As mentioned above, it is found that Al may diffuse from the workfunction metal layers 226 a and 226 b. As shown in FIG. 21 , because thesecond work function metal layer 226 b is in contact with the secondhigh-k gate dielectric layer 222 b, Al may diffuse into the secondhigh-k gate dielectric layer 222 b. Consequently, the second high-k gatedielectric layer 222 b may include metal material, such as Al.Similarly, Al may diffuse from the first work function metal layer 226 aand from the third work function metal layer 226 c. However, the firstbarrier layer 224 a and the third barrier layer 224 c may mitigate theAl diffusion due to the introduction of nitrogen. As mentioned above,nitrogen in the N-containing barrier layers 224 a and 224 c helps toobstruct Al diffusion even in such thin layers. Therefore, Al diffusioninto the first high-k gate dielectric layer 222 a and the third high-kgate dielectric layer 222 c is less than that into the second high-kgate dielectric layer 222 b. Further, the first barrier layer 224 a mayhave greater Al diffusion barrier ability due to its greater Nconcentration. In some embodiments, the first high-k gate dielectriclayer 222 a, the second high-k gate dielectric layer 222 b and the thirdhigh-k gate dielectric layer 222 c may still include the metal materialsuch as Al, but a metal concentration (i.e., the Al concentration) ofthe third high-k gate dielectric layer 222 c is less than a metalconcentration (i.e., the Al concentration) of the second high-k gatedielectric layer 222 b, and a metal concentration (i.e., the Alconcentration) of the first high-k gate dielectric layer 222 a is lessthan the metal concentration (i.e., the Al concentration) of the thirdhigh-k gate dielectric layer 222 c.

It should be noted that the second high-k gate dielectric layer 222 bwith high Al concentration is suitable for an LV device, the firsthigh-k gate dielectric layer 222 a with low Al concentration is suitablefor an HV device, and the third high-k gate dielectric layer 222 c withmedium Al concentration is suitable for an MV device. Accordingly, thesemiconductor structure 20′ is a multiple-Vt structure suitable fordevice design.

Accordingly, the method 10 includes forming the N-containing barrierlayers 224 a and 224 c with different N concentration by in-situ andex-situ treatment for the HV device. The N-containing barrier layers 224a and 224 c are able to mitigate the Al diffusion, therefore such thinbarrier layers have less impact on gap filling. In short, the method 10provides the N-containing barrier for meeting multiple-Vt structurerequirements with competitive gap-filling ability.

In summary, the present disclosure provides a method for forming asemiconductor structure. The method may be integrated into an HKMGprocess. The method may also be integrated with formation of a planardevice or a non-planar device. In some embodiments, the method forforming the semiconductor structure includes in-situ and/or ex-situnitridation of barrier layers. The barrier layers with differentnitrogen concentrations may have different metal barrier abilities,thereby helping the semiconductor structure to meet multiple-Vtstructure requirements.

Some embodiments of the present disclosure provide a method for forminga semiconductor structure. The method includes following operations. Afirst FET device and a second FET device are formed over a substrate. Afirst gate trench is formed in the first FET device, and a second gatetrench is formed in the second FET device. A first high-k gatedielectric layer is formed in the first gate trench, and a second high-kgate dielectric layer is formed in the second gate trench. A firstbarrier layer is formed over the first high-k gate dielectric layer, anda second barrier layer is formed over the second high-k gate dielectriclayer. An N concentration in the first barrier layer and an Nconcentration in the second barrier layer are increased. The secondbarrier layer is removed to expose the second high-k gate dielectriclayer. A first work function metal layer is formed over the firstbarrier layer, and a second work function metal layer is formed over thesecond high-k gate dielectric layer.

Some embodiments of the present disclosure provide a method for forminga semiconductor structure. The method includes following operations. Afirst FET device, a second FET device and a third FET device are formedover a substrate. A first gate trench is formed in the first FET device,a second gate trench is formed in the second FET device, and a thirdgate trench is formed in the third FET device. A first high-k gatedielectric layer is formed in the first gate trench, a second high-kgate dielectric layer is formed in the second gate trench, and a thirdhigh-k gate dielectric layer is formed in the third gate trench. A firstbarrier layer is formed over the first high-k gate dielectric layer, asecond barrier layer is formed over the second high-k gate dielectriclayer, and a third barrier layer is formed over the third high-k gatedielectric layer. A first nitridation is performed on the first barrierlayer, the second barrier layer and the third barrier layer. A secondnitridation is performed on the first barrier layer. The second barrierlayer is removed to expose the second high-k gate dielectric layer inthe second gate trench. A first work function metal layer is formed overthe first barrier layer, a second work function metal layer is formedover the second high-k gate dielectric layer, and a third work functionmetal layer is formed over the third barrier layer.

Some embodiments of the present disclosure provide a method for forminga semiconductor structure. The method includes following operations. Afirst FET device, a second FET device and a third FET device are formedover a substrate. A first gate trench is formed in the first FET device,a second gate trench is formed in the second FET device, and a thirdgate trench is formed in the third FET device. A first high-k gatedielectric layer is formed in the first gate trench, a second high-kgate dielectric layer is formed in the second gate trench, and a thirdhigh-k gate dielectric layer is formed in the third gate trench. A firstbarrier layer is formed over the first high-k gate dielectric layer, asecond barrier layer is formed over the second high-k gate dielectriclayer, and a third barrier layer is formed over the third high-k gatedielectric layer. An N concerntration of the first barrier layer, an Nconcentration of the secon barrier layer and an N concentration of thethird barrier layer are increased. A protecting layer is formed over thesecond barrier layer and the third barrier layer. The N concentration ofthe first barrier layer is increased. The protecting layer is removed.The second barrier layer is removed to expose the second high-k gatedielectric layer in the second gate trench. A first work function metallayer is formed over the first barrier laer, a second work functionmetal layer is formed over the secnd high-k gate dielectric layer, and athird work function metal layer is formed over the third barrier layer.The first high-k gate dielectric layer, the second high-k gatedielectric layer and the third high-k gate dielectric layer include asame metal material.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A method for forming a semiconductor structurecomprising: forming a first field effect transistor (FET) device and asecond FET device over a substrate; forming a first gate trench in thefirst FET device and a second gate trench in the second FET device;forming a first high-k gate dielectric layer in the first gate trenchand a second high-k gate dielectric layer in the second gate trench;forming a first barrier layer over the first high-k gate dielectriclayer and a second barrier layer over the second high-k gate dielectriclayer; increasing an N concentration in the first barrier layer and an Nconcentration in the second barrier layer; removing the second barrierlayer to expose the second high-k gate dielectric layer in the secondgate trench; and forming a first work function metal layer over thefirst barrier layer and a second work function metal layer over thesecond high-k gate dielectric layer.
 2. The method of claim 1, whereinthe increasing of the N concentration comprises an in-situ treatmentand/or an ex-situ treatment.
 3. The method of claim 2, wherein thein-situ treatment comprises an ammonia (NH₃) pulse and/or an ammoniasoak.
 4. The method of claim 2, wherein the ex-situ treatment comprisesNH₃, nitrogen (N₂), triatomic hydrogen (H₃), inert gas, or combinationsthereof.
 5. The method of claim 1, wherein the N concentrations of thefirst barrier layer and the second barrier layer are increased toapproximately 10% to approximately 70%.
 6. The method of claim 1,wherein the first high-k gate dielectric layer and the second high-kgate dielectric layer comprise a same metal material, the first high-kgate dielectric layer has a first metal concentration, the second high-kgate dielectric layer has a second metal concentration, and the firstmetal concentration is less than the second metal concentration.
 7. Themethod of claim 6, wherein the metal material comprises aluminum (Al).8. A method for forming a semiconductor structure comprising: forming afirst field effect transistor (FET) device, a second FET device and athird FET device over a substrate; forming a first gate trench in thefirst FET device, a second gate trench in the second FET device, and athird gate trench in the third FET device; forming a first high-k gatedielectric layer in the first gate trench, a second high-k gatedielectric layer in the second gate trench, and a third high-k gatedielectric layer in the third gate trench; forming a first barrier layerover the first high-k gate dielectric layer, a second barrier layer overthe second high-k gate dielectric layer, and a third barrier layer overthe third high-k gate dielectric layer; performing a first nitridationon the first barrier layer, the second barrier layer and the thirdbarrier layer; performing a second nitridation on the first barrierlayer; removing the second barrier layer to expose the second high-kgate dielectric layer in the second gate trench; and forming a firstwork function metal layer over the first barrier layer, a second workfunction metal layer over the second high-k gate dielectric layer, and athird work function metal layer over the third barrier layer.
 9. Themethod of claim 8, wherein an N concentration of the first barrierlayer, an N concentration of the second barrier layer and an Nconcentration of the third barrier layer are increased by the firstnitridation.
 10. The method of claim 9, wherein the N concentrations ofthe first barrier layer is increased by the second nitridation.
 11. Themethod of claim 8, wherein the N concentration of the first barrierlayer is greater than the N concentration of the second barrier layerand the N concentration of the third barrier layer.
 12. The method ofclaim 8, further comprising: forming a protecting layer over the secondbarrier layer and the third barrier layer prior to the performing of thesecond nitridation; and removing the protecting layer after theperforming of the second nitridation.
 13. The method of claim 8, whereinthe first high-k gate dielectric layer, the second high-k gatedielectric layer and the third high-k gate dielectric layer comprise asame metal material.
 14. The method of claim 13, wherein the metalmaterial comprises aluminum (Al).
 15. The method of claim 13, whereinthe first high-k gate dielectric layer has a first metal concentration,the second high-k gate dielectric layer has a second metalconcentration, the third high-k gate dielectric layer has a third metalconcentration, the first metal concentration and the third metalconcentration are less than the second metal concentration.
 16. Themethod of claim 15, wherein the first metal concentration is less thanthe third metal concentration.
 17. A method for forming a semiconductorstructure comprising: receiving a substrate comprising a first FETdevice, a second FET device and a third FET device; forming a first gatetrench in the first FET device, a second gate trench in the second FETdevice and a third gate trench in the third FET device; forming a firsthigh-k gate dielectric layer in the first gate trench, a second high-kgate dielectric layer in the second gate trench, and a third high-k gatedielectric layer in the third gate trench; forming a first barrier layerover the first high-k gate dielectric layer, a second barrier layer overthe second high-k gate dielectric layer, and a third barrier layer overthe third high-k gate dielectric layer; increasing an N concentration ofthe first barrier layer, an N concentration of the second barrier layerand an N concentration of the third barrier layer; forming a protectinglayer over the second barrier layer and the third barrier layer;increasing the N concentration of the first barrier layer; removing theprotecting layer; removing the second barrier layer to expose the secondhigh-k gate dielectric layer in the second gate trench; and forming afirst work function metal layer over the first barrier layer, a secondwork function metal layer over the second high-k gate dielectric layer,and a third work function metal layer over the third barrier layer,wherein the first high-k gate dielectric layer, the second high-k gatedielectric layer and the third high-k gate dielectric layer comprise asame metal material.
 18. The method of claim 17, wherein the metalmaterial comprises aluminum (Al).
 19. The method of claim 17, whereinthe first high-k gate dielectric layer has a first metal concentration,the second high-k gate dielectric layer has a second metalconcentration, the third high-k gate dielectric layer has a third metalconcentration, the first metal concentration and the third metalconcentration are less than the second metal concentration.
 20. Themethod of claim 19, wherein the first metal concentration is less thanthe third metal concentration.